408 Professors Perry and Ayrton on a new 



x is the number of cells necessary to produce our required 

 distribution, 



4tt x 0-0107 x ^/ 2 x 1Q9 10 7 



x= 



630 

 = 54 million roughly. 



We have therefore proved that if the earth be electrified, it 

 must, from its very rotation, quite independently of all other 

 bodies in the universe, be magnetic ; and if it consist of a 

 shell of iron, thick or thin, then that the law of distribution 

 of magnetism produced by this electric charge in mechanical 

 rotation will be identically that given by Biot ; and, lastly, 

 if the earth were wholly of iron, a difference of potentials of 

 about fifty-four million volts between it and space would be 

 sufficient to produce the necessary amount of charge. 



Now, although fifty-four million volts is a large difference 

 of potentials to be produced with a galvanic battery, there 

 would not be the slightest difficulty in the earth having such 

 a difference of potentials between it and space, seeing that the 

 earth is surrounded by millions of miles of interplanetary va- 

 cuum, every inch of which is as good or better an insulator 

 than a Crookes's vacuum; and it has been experimentally 

 shown that many thousands of cells will not cause a discharge 

 across even a comparatively thin film of such a vacuum. 



But even without considering the highly insulating cha- 

 racter of interplanetary space, we see from the experiments of 

 Drs. De La Rue and Hugo Muller that the electromotive force 

 of fifty-four million cells cannot, in all probability, initiate a 

 spark between two points in ordinary air unless the distance 

 separating them be Jess than four hundred feet. Consequently, 

 if the electric charge which by its mechanical rotation pro- 

 duces the magnetism be on the earth itself and not in the air, 

 it could not be discharged by sparking, unless another planet, 

 having at least a potential nought relatively to the earth, came 

 at least to within four hundred feet of its surface. 



Next, as regards the sign of the electric charge on the 

 earth's surface required to produce the earth's magnetic pola- 

 rity, is it in accordance with the known phenomena of atmo- 

 spheric electricity ? To produce the earth's magnetism, we 

 must have, in accordance with the known laws of electro- 

 magnetism, a negative current flowing from west to east, or in 

 in the direction of rotation of the earth. In the language of 

 the new theory, therefore, the surface of the earth must be 

 negatively charged ; but Sir William Thomson has proved, by 



